In deep sea oil or gas production sites, subsea power grids are installed for providing the required power for electrical actuator like electric motors and electric pumps. For controlling an electric motor, voltage, current and AC frequency may be varied. To this end, a variable speed drive (VSD) may be provided in the power grid. These variable speed drives contain inter alia DC link capacitors. The current design depths of the subsea variable speed drive are 1-3000 meters, which gives a surrounding sea water pressure of approximately 1-300 bar. Applications at even higher water depths up to 5000 m are foreseeable.
Furthermore, subsea power electronic converters are used in this context, wherein the converters may be used for electromagnetic mapping of the sea bed. The converters may be filled with dielectric fluid and are pressure compensated such that the internal fluid pressure inside the converter enclosure is almost equal to the pressure outside, i.e. the pressure of the surrounding sea water.
The converter comprises various electronic components such as a transformer, a diode rectifier, DC link capacitor, IGBT (insulated gate bipolar transistor), inverter, control systems. It has been found that the capacitor of such a converter is especially sensitive to the conditions of the surrounding dielectric fluid. A long lifetime of the capacitor in a subsea environment is crucial for the functioning of the converter.